This invention relates in general to multi-stage servovalves and in particular to a two-stage, shear-type, fail-fixed servovalve with motion amplification in its second stage and a method for actuating the second stage.
Servovalves are used broadly to interface between an electrical control system and mechanical metering or actuating devices, for example, as engine control equipment in an aircraft flight control system for controlling the fuel flow to a gas turbine in response to an electrical control signal. In the latter case, a control signal typically controls the operation of the servovalve such that the velocity of a servopiston changes with the control signal. The servopiston itself may be mechanically coupled to a fuel metering valve or the like, whose status determines the fuel flow to the engine. In such an arrangement, it is desirable to use a fail-fixed servovalve, i.e., a valve which causes the servopiston to lock in place immediately under certain conditions. In one case, the piston must be locked when a loss of the electrical control signal occurs in order to safeguard against unwanted change in fuel flow to the engine. In another instance, the piston must be locked when the electrical control signal exceeds a predetermined value in either direction in order to prevent unwanted fuel flow change if and when a malfunction occurs in the electrical control system.
In a two-stage electrohydraulic servovalve, a fluid under pressure is used to move a mechanical element, e.g. a spool or a piston. In the second stage, the pressurized fluid is vented to a servopiston chamber in accordance with the position of the mechanical element. One type of currently used two-stage servovalve, e.g. as shown in U.S. Pat. No. 4,227,443, further includes a torque motor in the first stage which moves a jet nozzle in response to an electrical control signal. The nozzle directs the flow of fluid toward a pair of input orifices, each receiving fluid for a separate fluid path. The two fluid paths terminate at opposite ends of a common bore in a housing. A spool is movably disposed in the bore such that the spool's position is controlled by the relative flow of fluid through the two fluid paths. A feedback spring has one end attached to the spool and the other end affixed to the nozzle. The spring repositions the jet pipe when the spool attains a position corresponding to the reference value of the control signal. The spool has a plurality of relieved areas interspaced with a plurality of lands. The housing contains passages which communicate between the bore and a high pressure fluid reservoir and a low pressure fluid sump respectively, and between the bore and opposite ends of a servopiston chamber. A servopiston, movably disposed within a servopiston chamber, is actuated by pressurized fluid vented through the aforesaid passages as the spool moves within the bore of the housing opening and closing the passages.
It is commonplace to actuate the first stage of the servovalve with either a DC control signal or a pulse width modulated control signal, provided the latter's frequency is high enough so that the torque motor, in the first stage, does not respond to each individual waveform applied to its inputs. Thus, a DC control signal of one-half of the maximum rated current applied to the first stage actuates that stage in a similar fashion as a pulse width modulated control signal which has an average value of one-half of the rated current if the frequency of the latter signal is high enough. The displacement and actual position of the spool, in the above-noted patent, is normally directly proportional to the time average torque motor current from a null or reference current. A detailed description of this spool-type servovalve appears in U.S. Pat. No. 4,227,443, which is assigned to the assignee herein and is incorporated herein by reference.
The spool-type servovalve, as described above, requires a spool which is closely fitted to the bore of the housing to prevent significant leakage around the spool when the spool blocks the passages between the high and low pressure reservoirs and the servopiston chamber. Even with a close fit, the rate at which fluid seeps between the spool and the bore is not constant. Therefore when the spool closes the passages leading to the servopiston chamber, the leakage around the spool causes the servopiston to move slightly or creep at a rate which is difficult to predict or to maintain constant. When the spool and bore wear with use, the creep is even more difficult to determine. The requirement for a close fit makes the spool-type servovalve costly to manufacture since the spool must be machined to fit the bore precisely. Spool-type servovalves are also vulnerable to contaminant material, such as grit in the fluid supply, which is capable of jamming the spool in the bore.
Electrohydraulic two-stage servovalves currently in use in aircraft flight control systems commonly operate in a closed system at hydraulic supply pressures on the order of 3,000 psi. In a closed system, the hydraulic fluid, usually oil, is recirculated and fine-filtered. Since the fluid is exposed to very few elements external to the system, few external contaminants are introduced into the system and the amount of grit, or similar contaminate material in the fluid which may affect the operation of the servovalve, can be kept to a minimum.
In an engine control system, the engine fuel itself is preferably used as the hydraulic fluid. Such a system is by necessity an open system and hence fine-filtering requires large or multiple filters or frequent filter element replacement. An engine control system of this type should therefore be tolerant of comparatively large amounts of contaminant. Further, such a control system normally operates at a lower pressure, i.e., in the range from 200 to 1,000 psi. Hence, the servovalves of such a control system have smaller second stage force levels or contaminant shearing force levels than those of a high pressure system, unless the spools are made larger.